修改和传递全局变量

Question

I'm having trouble modifying a global variable in a callback.

I need to reload the data from a file every hour. It works fine if I use readFileSync but not if I use the normal readFile :

var MY_GLOBAL = {};

fs.readFile("some_path", function (err, data) {
    if (err) throw err;
    MY_GLOBAL.some_name = data;
    });

setInterval(function() {
    fs.readFile("some_path", function (err, data) {
        if (err) throw err;
        MY_GLOBAL.some_name = data;
        }
    }, 60 * 60 * 60 * 1000);

I have another module named router.js which contains all the routing for my application.

I call it using: router(app, MY_GLOBAL) .

My issue is that although MY_GLOBAL gets set in the readFile callback, it doesn't update in the router. I need to access the updated data every hour.

Answer 1

I can suggest several things to check.

First, I would double check that the callback function is executing for certain using console.log or alert .

I would also like to point out that objects are passed by reference in Javascript, which means that the object passed in to router is a reference to MY_GLOBAL , not a copy of MY_GLOBAL . Where this can go wrong is if somewhere you reassigned MY_GLOBAL. I would make sure that you did not reassign MY_GLOBAL anywhere.

Also, if data is an object (as opposed to a string, number, or boolean), you need to check that it doesn't get reassigned, either. For example, if you read the value of MY_GLOBAL.some_name inside of router , it is possible that you could retain a reference to an old version after data is assigned to MY_GLOBAL.some_name.

Answer 2

If its a global statement why not try to use the module keyword such a lot of people does with the window .

another thing. You want the function code to be executed every hour. you are putting one extra 60 multiplication into the second parameter. try this.

module.MY_GLOBAL = {};

var callbackFunction = function(){
  fs.readFile("some_path", function (err, data) {
    if (err) throw err;
    module.MY_GLOBAL.some_name = data;
  }
};
callbackFunction();
setInterval(callbackFunction , 60 * 60 * 1000); // 360000 == 1 hour